• better performance, eliminated need for nested locks and
  thread privates. Moving request setups from the main connection to the
  worker threads. Increase number of spare connections kept.
• input buffering and dynamic flow windows for increased
  throughput. Requires nghttp2 >= v1.5.0 features. Announced at startup
  in mod_http2 INFO log as feature 'DWINS'.
• h2 workers with improved scalability for better scheduling
  performance. There are H2MaxWorkers threads created at start and the
  number is kept constant for now.
• obsoleted option H2SessionExtraFiles, will be ignored and
  just log a warning.
• fixed PR60869 by making h2 workers exit explicitly waking up
  all threads to exit in a defined way.